1. Field of the Invention
The present invention relates the lamp arts. It finds particular application in conjunction with improvement in the performance of tungsten halogen lamps by reducing oxygen and/or moisture content of the lamp.
2. Discussion of the Art
Tungsten halogen lamps, which comprise a hermetically sealed vitreous, light transmissive envelope enclosing a tungsten filament and containing a fill comprising a halide or halogen gas are widely used in a variety of applications. These lamps operate on a tungsten halogen cycle which is a regenerative, continuous process in which a halogen-containing tungsten compound is produced when the halide combines chemically with particles of tungsten evaporating from the incandescent tungsten filament. Subsequent thermal decomposition of these so-formed halogen-containing tungsten compounds at the filament returns the tungsten particles back to the filament. Halogen compounds used for the fill include bromine and bromides, such as hydrogen bromide, methyl bromide, dibromomethane, and bromoform. For lamps that operate at low temperatures, i.e. below about 200° C. interior wall temperatures, premature failure of the lamps may occur due to blackening of the glass envelope. The blackening is due to tungsten transferred from the filament to the wall. The presence of oxygen and/or water vapor in the lamp atmosphere has been found to contribute to the wall blackening. Water vapor is particularly harmful because even trace amounts increase the evaporation of the tungsten filament coil by means of the well-known “water cycle.” In the water cycle, the temperature of the tungsten coil is thermally sufficient to decompose water vapor into hydrogen and oxygen. The resulting oxygen reacts with the tungsten from the coil to form volatile oxides, which migrate to cool parts of the lamp and condense. These oxide deposits are reduced by the gaseous hydrogen to yield black metallic tungsten and reformed water, which causes the cycle to repeat.
Excess oxygen in incandescent lamps is similarly a problem. For example in the tungsten halogen cycle, oxygen is the primary agent of attack on the tungsten filament. This attack may result in etching and oxide growth and usually causes early filament failure. However, small amounts of oxygen have been found to promote the tungsten cycle by reacting with the tungsten on the walls to form a compound of tungsten, oxygen, and halogen. This gaseous compound moves back to the filament and decomposes back to tungsten at the filament.
Methods of reducing excess oxygen in tungsten halogen lamps are well known. One commonly utilized solution is to introduce one or more compounds into the lamp which will remove excess oxygen and prevent its participation in the tungsten halogen cycle. Such compounds are commonly referred to as oxygen getters. Oxygen gettering systems which have been used previously include metallic getters such as tantalum, zirconium, niobium, copper, titanium, aluminum, and various combinations of these metals. The fabrication of specialized alloys can add considerably to the cost of manufacturing the lamp. Additionally, such getters tend to function only in discreet locations within the lamp.
Phosphorus compounds have also been used as oxygen getters, including phosphine gas, which is thermally decomposed into phosphorus and hydrogen in the energized lamp. Another gettering system comprises silane compounds, as disclosed in U.S. Pat. No. 4,898,558. Activation of these silane getters is achieved by baking the filled lamp at a temperature of about at least 350° C. for five minutes to react the silane with any oxygen in the lamp to form non-volatile silicon dioxide and hydrogen. Such lamps have been found to have reduced lifetimes, as compared with silane-free equivalents. Recently, tungsten halogen lamps have been used in automotive products, such as lights. To achieve a white light emission from the generally yellow tungsten halogen lamps, it has been found that the application of a blue coating can be used. However, because the coating absorbs or reflects a portion of the emitted light, the lumen loss may be up to 5–30%. To compensate for the lumen loss it is desired to produce a filament with increased lumens to compensate for the lumen loss from the blue absorption coating. Although higher wattages may used to compensate for the lumen loss, for certain products, such as automotive products the option of increasing wattage is not available. In addition, higher wattages generate more heat and hence increase the probability of bulb wall darkening due to the higher bulb wall temperature, with reduced efficiency of the tungsten-halogen transport cycle. For such lamps, it is desirable to provide lumen output comparable to conventional lamps, while maintaining efficiency over the lifetime of the lamp.
The present invention provides a new and improved lamp and method of preparation, which overcomes the above-referenced problems and others.